This research examines how the development of a sensory system is reflected in the morphology of neurons and the organization of pathways. In amphibia, the ear structures which transduce sound appear during metamorphosis. In Rana, the associated auditory input reaches the brain before the disappearance of lateral line afferent fibers and projects to a medullary nucleus which is being generated during the same period. This nucleus, the incipient acoustic nucleus (AcN), organizes to process and relay auditory information. Thus, the AcN represents an example of the formation of a functionally specialized system from a pool of newly generated neurons. Because it is experimentally accessible during the critical period of neuronal recruitment and at the time of afferentation, the AcN has great potential for studies of the development of sensory systems and their component elements. This project will investigate the process of sensory system development by describing the morphology of auditory pathways and nuclei, under normal conditions and following the removal of components of the lateral line and auditory input. The techniques to be employed include tritiated-thymidine labelling, HRP transport and experimental degeneration. The resulting data will be subjected to a computer-aided morphometric analysis of the relative distribution of cells, dendrites and terminal fields. These techniques will be applied to acoustic nuclei, superior olivary nuclei and torii semicirculares of Rana catesbeiana tadpoles during the critical periods of auditory development. The proposed studies are intended to gain insight into the plastic properties of the sensory system neurons. Application of sensory prostheses and post-surgical physiotherapy may rely upon such plastic properties.